Structure Of Gas-phase Ions Research Articles

Structural Studies of a Stapled Peptide with Native Ion Mobility-Mass Spectrometry and Transition Metal Ion Förster Resonance Energy Transfer in the Gas Phase.

Native mass spectrometry has emerged as an important tool for gas-phase structural biology. However, the conformations that a biomolecular ion adopts in the gas phase can differ from those found in solution. Herein, we report a synergistic, native ion mobility-mass spectrometry (IM-MS) and transition metal ion Förster resonance energy transfer (tmFRET)-based approach to probe the gas-phase ion structures of a nonstapled peptide (nsp; Ac-CAARAAHAAAHARARA-NH2) and a stapled peptide (sp; Ac-CXARAXHAAAHARARA-NH2). The stapled peptide contains a single hydrocarbon chain connecting the peptide backbone in the i and i + 4 positions via a Grubbs ring-closure metathesis. Fluorescence lifetime measurements indicated that the Cu-bound complexes of carboxyrhodamine 6g (crh6g)-labeled stapled peptide (sp-crh6g) had a shorter donor-acceptor distance (rDA) than the labeled nonstapled peptide (nsp-crh6g). Experimental collision cross-section (CCS) values were then determined by native IM-MS, which could separate the conformations of Cu-bound complexes of nsp-crh6g and sp-crh6g. Finally, the experimental CCS (i.e., shape) and rDA (i.e., distance) values were used as constraints for computational studies, which unambiguously revealed how a staple reduces the elongation of the peptide ions in the gas phase. This study demonstrates the superiority of combining native IM-MS, tmFRET, and computational studies to investigate the structure of biomolecular ions.

UV‐vis spectroscopy of gas‐phase ions

Photodissociation action spectroscopy has made a great progress in expanding investigations of gas-phase ion structures. This review deals with aspects of gas-phase ion electronic excitations that result in wavelength-dependent dissociation and light emission via fluorescence, chiefly covering the ultraviolet and visible regions of the spectrum. The principles are briefly outlined and a few examples of instrumentation are presented. The main thrust of the review is to collect and selectively present applications of UV-vis action spectroscopy to studies of stable gas-phase ion structures and combinations of spectroscopy with ion mobility, collision-induced dissociation, and ion-ion reactions leading to the generation of reactive intermediates and electronic energy transfer.

Reevaluating the Role of Polarizability in Ion Mobility Spectrometry

With the expanding commercial availability of gas-phase separation systems that incorporate gas-phase mobility, there is a concurrent rise in efforts to cast the gas-phase mobility coefficient in terms of an ion-neutral collision cross-section (CCS). The motivating factors for this trend are varied, but many aim to complement experimental results with computationally generated CCS values from in silico structural approximations. Unfortunately, the current paradigm for relating experimental mobility results to computationally derived structures relies upon empirical approaches, including a myriad of variables that do not realistically bound the comparison. In this Critical Insight, we advocate for the development of a self-consistent experimental and computational framework that uses laboratory results to constrain the scope of the modeling effort. This paper aims to prompt discussion, challenge assumptions, and promote the development of more efficient, accurate computational techniques within the gas-phase ion measurement community. Specifically, we postulate whether experimental deviations from Langevin's polarization limit (Kpol) are suitable to estimate the relative contributions of hard-sphere collisions and long-range interactions within CCS values. Not surprisingly, different molecule classes exhibit different trends in the K/Kpol ratio when normalized for reduced mass, and the most common IMS calibrants (e.g., tune mix, polyalanine, tetraalkylammonium salts) follow different polarizability trends than many of the analytes probed in the literature. Succinctly, if gas-phase ion structure is largely invariant based upon the colliding neutral and newly developed experimental efforts can quantitatively capture ion polarizability, then modeling efforts describing a target analyte must be self-consistent as the collision neutral is changed in silico.

Fragmentation Behavior and Gas-Phase Structures of Cationized Glycosphingolipids in Ozone-Induced Dissociation Mass Spectrometry.

The role of cationization in the fragmentation behavior of glycoconjugates is amply documented in collisional activation techniques but remains less explored in ozone-induced dissociation mass spectrometry (OzID-MS). OzID-MS has been used to elucidate the location of carbon-carbon double bonds in unsaturated lipids. Previously, we demonstrated the structural analysis of unsaturated glycosphingolipids using OzID-MS by mass-selecting the [M+Na]+ adduct for fragmentation. In this work, we aimed to examine the effect of different adducts, namely [M+Na]+, [M+Li]+, and [M+H]+ on the OzID-MS fragmentation behavior of a representative unsaturated glycosphingolipid, LacCer d18:1/18:1(9Z). Our data show that [M+H]+ primarily undergoes dehydration followed by collision-induced dissociation-like loss of the headgroup, while [M+Li]+ and [M+Na]+ dissociate at the double bonds albeit with slightly different intensities of the resulting fragments. Using molecular mechanics and theoretical calculations at the semiempirical level, we report for the first time the gas-phase structure of cationized glycosphingolipids, which helps rationalize the observed bond cleavage. Our findings highlight that the type of adducts can influence gas-phase ion structure of glycosphingolipids and subsequently affect their fragmentation in OzID-MS. This study contributes to the growing body of knowledge on OzID-MS and gas-phase structures of ionized lipids and the findings have the potential to be extended to other more complex glycoconjugates.

Determination of Gas-Phase Ion Structures of Locally Polar Homopolymers Through High-Resolution Ion Mobility Spectrometry-Mass Spectrometry.

The strong synergy arising from coupling two orthogonal analytical techniques such as ion mobility and mass spectrometry can be used to separate complex mixtures and determine structural information of analytes in the gas phase. A tandem study is performed using two systems with different gases and pressures to ascertain gas-phase conformations of homopolymer ions. Aside from spherical and stretched configurations, intermediate configurations formed by a multiply charged globule and a "bead-on-a-string" appendix are confirmed for polyethylene-glycol (PEG), polycaprolactone (PCL), and polydimethylsiloxane (PDMS). These intermediate configurations are shown to be ubiquitous for all charge states and masses present. For each charge state, configurations evolve in two distinctive patterns: an inverse evolution which occurs as an elementary charge attached to the polymer leaves the larger globule and incorporates itself into the appendage, and a forward evolution which reduces the globule without relinquishing a charge while leaving the appendix relatively constant. Forward evolutions are confirmed to form self-similar family shapes that transcend charge states for all polymers. Identical structural changes occur at the same mass over charge regardless of the system, gas or pressure strongly suggesting that conformations are only contingent on number of charges and chain length, and start arranging once the ion is at least partially ejected from the droplet, supporting a charge extrusion mechanism. Configurational changes are smoother for PDMS which is attributed to the larger steric hindrance caused by protruding pendant groups. This study has implications in the study of the configurational space of more complex homopolymers and heteropolymers. Graphical Abstract.

Influence of 2′-fluoro modification on glycosidic bond stabilities and gas-phase ion structures of protonated pyrimidine nucleosides

The effects of 2′-fluoro substitution on the protonated gas-phase ions of the pyrimidine nucleosides are examined and compared with their previously reported canonical RNA and DNA forms. N-Glycosidic bond cleavage is the only collision-induced dissociation (CID) fragmentation pathway of protonated 2′-fluoro-2′-deoxycytidine, [Cydfl+H]+, and the major pathway of protonated 2′-fluoro-2′-deoxyuridine, [Urdfl+H]+. Based on energy-resolved CID and survival yield analysis, the N-glycosidic bond of [Cydfl+H]+ is more stable than that of [Urdfl+H]+. Further, the N-glycosidic bond stability of protonated pyrimidine nucleosides increases with increasing 2′-substituent electronegativity and follows the order F > OH > H. Gas-phase conformations of [Cydfl+H]+ and [Urdfl+H]+ are studied via infrared multiple photon dissociation (IRMPD) action spectroscopy coupled with theoretical calculations. IRMPD action spectra are measured over the IR fingerprint and hydrogen-stretching regions. Comparisons of theoretical and experimental spectra indicate that the experimentally accessed [Cydfl+H]+ and [Urdfl+H]+ conformers are highly parallel to those populated for their canonical counterparts. Evidence for gas-phase intramolecular O3′H⋯F2′ hydrogen-bonding in the IRMPD spectra of [Cydfl+H]+ and [Urdfl+H]+ allows for more definitive sugar puckering determinations than possible in the analogous canonical nucleosides.

Effect of charge on the conformation of highly basic peptides including the tail regions of histone proteins by ion mobility mass spectrometry

The first systematic and comprehensive study of the charging behaviour and effect of charge on the conformation of specifically constructed arginine-rich peptides and its significance to the N- and C-terminal basic tail regions of histone proteins was conducted using ion mobility mass spectrometry (IM-MS). Among the basic amino acids, arginine has the greatest impact on the charging behaviour and structures of gas phase ions by virtue of its high proton affinity. A close linear correlation was found between either the maximum charge, or most abundant charge state, that the peptides support and their average collision cross section (CCS) values measured by ion mobility mass spectrometry. The calculated collision cross sections for the lowest energy solution state models predicted by the PEP-FOLD algorithm using a modified MOBCAL trajectory method were found to best correlate with the values measured by IM-MS. In the case of the histone peptides, more compact folded structures, supporting less than the maximum number of charges, were observed. These results are consistent with those previously reported for histone dimers where neutralization of the charge at the basic residues of the tail regions did not affect their CCS values.

Toward a Rational Design of Highly Folded Peptide Cation Conformations. 3D Gas-Phase Ion Structures and Ion Mobility Characterization.

Heptapeptide ions containing combinations of polar Lys, Arg, and Asp residues with non-polar Leu, Pro, Ala, and Gly residues were designed to study polar effects on gas-phase ion conformations. Doubly and triply charged ions were studied by ion mobility mass spectrometry and electron structure theory using correlated ab initio and density functional theory methods and found to exhibit tightly folded 3D structures in the gas phase. Manipulation of the basic residue positions in LKGPADR, LRGPADK, KLGPADR, and RLGPADK resulted in only minor changes in the ion collision cross sections in helium. Replacement of the Pro residue with Leu resulted in only marginally larger collision cross sections for the doubly and triply charged ions. Disruption of zwitterionic interactions in doubly charged ions was performed by converting the C-terminal and Asp carboxyl groups to methyl esters. This resulted in very minor changes in the collision cross sections of doubly charged ions and even slightly diminished collision cross sections in most triply charged ions. The experimental collision cross sections were related to those calculated for structures of lowest free energy ion conformers that were obtained by extensive search of the conformational space and fully optimized by density functional theory calculations. The predominant factors that affected ion structures and collision cross sections were due to attractive hydrogen bonding interactions and internal solvation of the charged groups that overcompensated their Coulomb repulsion. Structure features typically assigned to the Pro residue and zwitterionic COO-charged group interactions were only secondary in affecting the structures and collision cross sections of these gas-phase peptide ions. Graphical Abstract ᅟ.

Size-to-charge dispersion of collision-induced dissociation product ions for enhancement of structural information and product ion identification.

Ion mobility is used to disperse product ions formed by collision-induced dissociation (CID) on the basis of charge state and size-to-charge ratio. We previously described an approach for combining CID with ion mobility mass spectrometry (IM-MS) for dispersing fragment ions along charge state specific trend lines (Zinnel, N. F.; Pai, P. J.; Russell, D. H. Anal. Chem. 2012, 84, 3390; Sowell, R. A.; Koeniger, S. L.; Valentine, S. J.; Moon, M. H.; Clemmer, D. E. J. Am. Soc. Mass Spectrom. 2004, 15, 1341; McLean, J. A.; Ruotolo, B. T.; Gillig, K. J.; Russell, D. H. Int. J. Mass Spectrom. 2005, 240, 301), and this approach was used to assign metal ion binding sites for human metallothionein protein MT-2a (Chen, S. H.; Russell, W. K.; Russell, D. H. Anal. Chem. 2013, 85, 3229). Here, we use this approach to distinguish b-type N-terminal fragment ions from both internal fragment ions and y-type C-terminal fragment ions. We also show that in some cases specific secondary structural elements, viz., extended coils or helices, can be obtained for the y-type fragment ions series. The advantage of this approach is that product ion identity can be correlated to gas-phase ion structure, which provides rapid identification of the onset and termination of extended coil structure in peptides.

Structure Elucidation of the Diagnostic Product Ion at m/z 97 Derived from Androst-4-en-3-One-Based Steroids by ESI-CID and IRMPD Spectroscopy

Structure elucidation of steroids by mass spectrometry has been of great importance to various analytical arenas and numerous studies were conducted to provide evidence for the composition and origin of (tandem) mass spectrometry-derived product ions used to characterize and identify steroidal substances. The common product ion at m/z 97 generated from androst-4-ene-3-one analogs has been subject of various studies, including stable isotope-labeling and (high resolution/high accuracy) tandem mass spectrometry, but its gas-phase structure has never been confirmed. Using high resolution/high accuracy mass spectrometry and low resolution tandem mass spectrometry, density functional theory (DFT) calculation, and infrared multiple photon dissociation (IRMPD) spectroscopy employing a free electron laser, the structure of m/z 97 derived from testosterone was assigned to protonated 3-methyl-2-cyclopenten-1-one. This ion was identified in a set of six cyclic C(6)H(9)O(+) isomers as computed at the B3LYP/6-311++G(2d,2p) level of theory (protonated 3-methyl-2-cyclopenten-1-one, 2-methyl-2-cyclopenten-1-one and 2-cyclohexen-1-one). Product ions of m/z 97 obtained from MS(2) and MS(3) experiments of protonated 3-methyl-2-cyclopenten-1-one, 2-methyl-2-cyclopenten-1-one, 2-cyclohexen-1-one, and testosterone corroborated the suggested gas-phase ion structure, which was eventually substantiated by IRMPD spectroscopy yielding a spectrum that convincingly matched the predicted counterpart. Finally, the dissociation pathway of the protonated molecule of testosterone to m/z 97 was revisited and an alternative pathway was suggested that considers the exclusion of C-10 along with the inclusion of C-5, which was experimentally demonstrated with stable isotope labeling.

Modelling of the gas-phase phosphate group loss and rearrangement in phosphorylated peptides

Phosphorylation is a key reversible protein posttranslational modifi cation that regulates enzymatic activity, subcellular localization, complex formation and degradation of proteins. Over the past several years, advances in mass spectrometry instrumentation coupled to the development of analytical methods have allowed investigation of the phosphoproteome on a global scale. The molecular modeling work presented in this special feature article provides insight into the energetic and kinetic aspects of the gas-phase unimolecular dissociation of phosphorylated peptides. During phosphorylation analysis by CID tandem MS, some gas-phase chemistry issues are causing diffi culties in proper sequence assignments, and consensus concerning the gas-phase fragmentation pathways for phosphate loss has not yet been reached. Therefore, elucidation of the proper phosphopeptide fragmentation behavior is of high importance in the context of predicting tandem MS ion abundances. Marko Rozman (Ph.D.) is a talented research associate in the Laboratory for chemical kinetics and atmospheric chemistry of the Ruder Boskovic Institue in Zagreb Croatia with strong interests in modeling gas phase ion structures and fragmentation reactions.

Gas-phase H/D-exchange reactions on resorcinarene and pyrogallarene capsules: Proton transport through a one-dimensional Grotthuss mechanism

Hydrogen/deuterium exchange (HDX) experiments can be used to examine the gas-phase structure of hydrogen-bonded dimeric resorcinarene and pyrogallarene capsules. Already the qualitative comparison of the isotope exchange rates of different host–guest complexes with Cs+, tetramethyl ammonium (TMA+) and tetraethyl ammonium (TEA+) as the guest cations provides insight into the H/D-exchange mechanisms and with it, into the capsules' gas-phase ion structures. The smaller Cs+cations bind inside dimeric capsules with an intact seam of hydrogen bonds between the two monomers. Larger cations such as TEA+ lead to capsules with partially disrupted seams of hydrogen bonds. A fast isotope exchange is only observed, when the H-bonding seam between the two monomers is intact. In these cases, the H/D-exchange proceeds by a concerted mechanism reminiscent of the Grotthuss mechanism of proton transfer through water. Since it can only proceed along the seam, we refer to this exchange mechanism as a one dimensional Grotthuss mechanism.

Machine learning based prediction for peptide drift times in ion mobility spectrometry.

Ion mobility spectrometry (IMS) has gained significant traction over the past few years for rapid, high-resolution separations of analytes based upon gas-phase ion structure, with significant potential impacts in the field of proteomic analysis. IMS coupled with mass spectrometry (MS) affords multiple improvements over traditional proteomics techniques, such as in the elucidation of secondary structure information, identification of post-translational modifications, as well as higher identification rates with reduced experiment times. The high throughput nature of this technique benefits from accurate calculation of cross sections, mobilities and associated drift times of peptides, thereby enhancing downstream data analysis. Here, we present a model that uses physicochemical properties of peptides to accurately predict a peptide's drift time directly from its amino acid sequence. This model is used in conjunction with two mathematical techniques, a partial least squares regression and a support vector regression setting. When tested on an experimentally created high confidence database of 8675 peptide sequences with measured drift times, both techniques statistically significantly outperform the intrinsic size parameters-based calculations, the currently held practice in the field, on all charge states (+2, +3 and +4). The software executable, imPredict, is available for download from http:/omics.pnl.gov/software/imPredict.php rds@pnl.gov Supplementary data are available at Bioinformatics online.

IR spectroscopy of cationized aliphatic amino acids: Stability of charge-solvated structure increases with metal cation size

Gas-phase structures of alkali metal cationized (Li +, Na +, K +, Rb +, and Cs +) proline (Pro) and N-methyl alanine have been investigated using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser and computational modeling. Measured IRMPD spectra are compared to spectra calculated at the B3LYP/6-311++G(2d,2p) level of theory to identify individual conformers. Calculations indicate that the stability of the salt bridge (SB; zwitterionic) conformer relative to the most stable canonical structure with a single formal charge site (charge solvation; CS) of aliphatic amino acids (e.g., Pro, N-methyl alanine, N-methyl glycine, and glycine) does not increase with size and polarizability of the alkali metal cations, in contrast to the trend commonly found for functionalized amino acids. In fact, the relative stability of SB over CS conformers reaches a maximum at [amino acid + Na] +. A uniform SB structure and two characteristic CS conformers are identified by theory to be relevant for alkali metalized Pro, N-methyl alanine, and N-methyl glycine. For CS structures, the alkali metal cation is either coordinated to the nitrogen and the carbonyl oxygen of the acid functionality (Li +, Na +) or is solely interacting with the carboxylic acid oxygens (K +, Rb +, and Cs +). The IRMPD spectra exhibit clearly distinguishable bands for the CO stretching modes of the carboxylic acid moiety in CS structures and for the carboxylate moiety in SB structures, allowing reliable structure assignments for all complexes investigated. The IRMPD spectra clearly exhibit the presence of mixed populations of SB and CS structures with the contribution of CS increasing toward the larger metal cations, in good agreement with the predictions from computational modeling. The special trend regarding formation and stability of individual gas-phase ion structures of aliphatic amino acids, lacking functionalized α-side chains, can be rationalized with the concept of hard and soft Lewis acids and bases. Furthermore, calculations show that the trends with metal cation size found for aliphatic amino acids with secondary amines are similar for ordinary aliphatic amino acids (Gly, Ala).

Structure Elucidation of Dimethylformamide-Solvated Alkylzinc Cations in the Gas Phase

Organozinc iodides, useful for the synthesis of nonproteinogenic amino acids, are investigated in the gas phase by a combination of electrospray (ESI)-MS/MS, accurate ion mass measurements, and infrared multiphoton dissociation (IRMPD) spectroscopy employing a free electron laser. ESI allowed the full characterization of a set of dimethylformamide (DMF)-solvated alkylzinc cations formed by formal loss of I(-) in the gas phase. Gas phase ion structures of the organozinc cations were identified and optimized by computations at the B3LYP/6-311G** level of theory. The calculations indicate that the zinc cation in gas phase alkylzinc-DMF species preferentially adopts a tetrahedral coordination sphere with four ligands, namely the alkyl group, any internal coordinating group, and DMF (the number of which depends on the number of internal coordinating groups present). Besides the sequential loss of coordinated DMF, collision-induced dissociation (CID) patterns demonstrate that the zinc-DMF interaction in tetrahedral four-coordinate mono-DMF-zinc complex ions can be even stronger than covalent bonds. The IRMPD spectra of the alkylzinc-DMF species examined show a rich pattern of indicative bands in the range of 1000-1800 cm(-1). All major features of the recorded IRMPD spectra are consistent with the computed IR spectra of the respective gas phase ion structures predicted by theory, allowing identification and assignment.

On the Structure Elucidation Using Ion Mobility Spectrometry and Molecular Dynamics

A new approach is described for the elucidation of gas-phase peptide ion structures combining ion mobility spectrometry (IMS) data and molecular dynamics (MD)-cluster analysis (CA) prediction. The new approach is based on the determination of the gas-phase ion structure identity vectors (e.g., structure and population vectors) that generate the total conformational space of the gas-phase ion as a function of the IMS experimental conditions (e.g., field strength, pressure, bath gas temperature, and IM cell geometry). Two methods to efficiently sample the gas-phase conformational space of molecular ions as a function of the effective ion temperature characteristic of the IMS experiments are described: (i) a simulated annealing MD-CA-constant temperature MD-CA, and (ii) a generalized non-Boltzmann sampling MD-free energy analysis-CA. The new theoretical method has been successfully applied to two model peptide ions (Bradykinin fragments 1-5 and 1-8, RPPGF and RPPGFSPF, respectively) for which multiple conformations sensitive to the effective ion temperature have been suggested in previous studies.

Optimization of Algorithms for Ion Mobility Calculations

Ion mobility spectrometry (IMS) is increasingly employed to probe the structures of gas-phase ions, particularly those of proteins and other biological macromolecules. This process involves comparing measured mobilities to those computed for potential geometries, which requires evaluation of orientationally averaged cross sections using some approximate treatment of ion-buffer gas collisions. Two common models are the projection approximation (PA) and exact hard-spheres scattering (EHSS) that represent ions as collections of hard spheres. Though calculations for large ions and/or conformer ensembles take significant time, no algorithmic optimization had been explored. Previous EHSS programs were dominated by ion rotation operations that allow orientational averaging. We have developed two new algorithms for PA and EHSS calculations: one simplifies those operations and greatly reduces their number, and the other disposes of them altogether by propagating trajectories from a random origin. The new algorithms were tested for a representative set of seven ion geometries including diverse sizes and shapes. While the best choice depends on the geometry in a nonobvious way, the difference between the two codes is generally modest. Both are much more efficient than the existing software, for example faster than the widely used Mobcal (implementing EHSS) approximately 10-30-fold.

Investigation of Gas Phase Ion Structure for Proline-Containing b 2 Ion

Unusual fragmentation was observed for doubly charged VPDPR in which cleavage C-terminal to proline and N-terminal to aspartic acid yielded b(2) (+ a(2))/y(3) complementary ions. This unique fragmentation is contradictory to trends previously established by statistical analysis of peptide tandem mass (MS/MS) spectra. Substitution of alanine for aspartic acid (i.e., VPAPR) did not change the fragmentation, indicating the cleavage was not directed by aspartic acid. Fragmentation patterns for VPAPR and V(NmA)APR (NmA = N-methyl alanine) were compared to determine whether conformational constraints from proline's cyclic side-chain contribute to b(2) ion formation. While both peptide sequences fragmented to yield b(2)/y(3) ions, only VPAPR produced a(2) ions, suggesting the VP b(2) ion is structurally different from the V(NmA) b(2) ion. Instead, the V(NmA) b(2) ion was accompanied by an ion corresponding to formal loss of 71. The suspected structural differences were confirmed by isolation and fragmentation of the respective b(2) ions (i.e., MS(3) spectra). Evidence supporting a diketopiperazine structure for the VP b(2) ion is reported. Fragmentation patterns for the VP b(2) ion and a synthetic VP diketopiperazine showed great similarity. N-terminal acetylation of VPAPR prevented the formation of the VP b(2) ion, presumably by blocking nucleophilic attack by the N-terminal amine on the carbonyl oxygen of the protonation site. Acetylation of the N-terminus for V(NmA)APR did not prevent the formation of the V(NmA) b(2) ion, indicating the V(NmA) b(2) ion has a structure, presumably that of an oxazolone, which requires no attack by the N-terminus for formation. Finally, high-resolution, accurate mass measurements determined that the V(NmA) (b(2)-71) ion results from losing a portion of valine from oxazolone V(NmA) b(2) ion, rather than cross-ring cleavage of the alternate diketopiperazine.

The Structure of Gas-Phase Bradykinin Fragment 1-5 (RPPGF) Ions: An Ion Mobility Spectrometry and H/D Exchange Ion-Molecule Reaction Chemistry Study

Ion mobility-mass spectrometry (IM-MS) data is interpreted as evidence that gas-phase bradykinin fragment 1-5 (BK1-5, RPPGF) [M + H](+) ions exist as three distinct structural forms, and the relative abundances of the structural forms depend on the solvent used to prepare the matrix-assisted laser desorption ionization (MALDI) samples. Samples prepared from organic rich solvents (90% methanol/10% water) yield ions having an ion mobility arrival-time distribution (ATD) that is dominated by a single peak; conversely, samples prepared using mostly aqueous solvents (10% methanol/90% water) yield an ATD composed of three distinct peaks. The BK1-5 [M + H](+) ions were also studied by gas-phase hydrogen/deuterium (H/D) exchange ion-molecule reactions and this data supports our interpretation of the IM-MS data. Plausible structures for BK1-5 ions were generated by molecular dynamics (MD). Candidate MD-generated structures correlated to measured cross-sections suggest a compact conformer containing a beta-turn whereas a more extended, open form does not contain such an interaction. This study illustrates the importance of intra-molecular interactions in the stabilization of the gas-phase ions, and these results clearly illustrate that solution-phase parameters (i.e., MALDI sample preparation) greatly influence the structures of gas-phase ions.

Application of self-ionization for enhancing stereochemical and positional effects from arylglycosides under electron ionization conditions in an ion trap mass spectrometer

Ion trap mass spectrometry has been used to structurally characterize and differentiate positional and stereo isomers of arylglycosides having potential antioxidant properties. The use of the self-ionization (SI) technique has allowed to evidence a strong reactivity of fragment ions produced from dissociations of the molecular ion towards the molecules introduced into the trap. Specific structural effects due to positional isomers and anomers have been also envisaged through the occurrence of bimolecular processes inside the ion trap analyzer. Under self-ionization conditions, even-electron ions are produced. The charge is retained on the sugar moiety, in agreement with its proton affinity higher than that of the substituted phenol moiety. Most of the fragmentation pathways involve elimination of acetic acid that protects the hydroxylic groups of the glycoside. SI also produces adduct ions, likely as covalent species, having higher m/ z values than the molecular ion. The reaction site is mainly the double bond present in the pyranosidic ring. Even if some fragment ions have lost the initial stereochemistry, their formation can be related to the structure of the parent neutrals introduced into the cell. Collision-induced dissociation (CID) experiments, carried out on ions formed by ion-molecule reactions, have allowed to obtain further information on gas phase ion structures. The study of mass-selected ion-molecule reactions and their kinetics have evidenced a spectacularly different reactivity of the ion at m/ z 111 towards the two anomers 2α and 2β, with the latter showing a much more pronounced reactivity. The approach developed in this work revealed to be an useful tool in structural characterization, as well as in stereo and regiochemical differentiation of arylglycosides.